1. Field of the Invention
The present invention relates to thermo-electric conversion elements which are constructed with a PN junction formed of a P type semiconductor and an N type semiconductor made of the new type of Si-based alloy system thermo-electric conversion material which has various alloying elements to a Si parent phase. More specifically, the present invention relates directly to a novel thermo-electric conversion element with which the thermo-electric conversion efficiency is improved by modifying the junction metal used for the PN junction portion and a joining metal employed between the semiconductors and lead wires.
2. Description of the Related Art
Thermo-electric conversion elements are currently in great demand for the effective application of a high thermal energy. Such devices can be found in systems that convert exhaust heat to electric energy, in small-scale portable power generators used outdoors, or in flame sensors which can be installed in equipment carrying and dealing with a gas.
However, it is generally believed that conventional thermo-electric conversion elements possess poor conversion efficiency. Moreover, the usable temperature range is relatively narrow, which is another drawback associated with conventional thermo-electric conversion elements. Furthermore, the production process is rather complicated, resulting in a high cost. These drawbacks make conventional conversion elements limited in their uses.
The efficiency in converting heat energy to electric energy can be expressed as a function of an efficiency index, ZT. Accordingly, when the index ZT is higher, the conversion efficiency will increase. The efficiency index, ZT, can be defined from the following equation (1); EQU ZT=.alpha..sup.2.sigma.T/K (1)
where the term ".alpha." is a Seebeck coefficient of the thermo-electric material, .sigma. is the electric conductivity, K is thermal conductivity, and T is an absolute temperature of the thermo-electric element which is averaged out over the high temperature side (T.sub.H) and low temperature side (T.sub.L).
A type of thermo-electric conversion element having the highest efficiency index is a Skutterudite type IrSb.sub.3 (T. Caillet, A. Borshchrysky and J. P. Fleurial: Proc. 12th Int. Conf. on Thermoelectrics, Yokohama Japan, 1993, page 132). It was reported that the ZT value of the Skutterudite IrSb.sub.3 is approximately 2.0. However, due to the extremely high cost of the raw material of the Ir element, this type of thermo-electric conversion element is not practical.
On the other hand, an Si--Ge alloy system and an Fe--Si alloy system are thought to be the most promising alloy systems from the viewpoints of cost and environmental considerations. However, although an Fe--Si alloy system possesses a relatively high value of Seebeck coefficient, the electric resistance is high and the efficiency index, ZT, is less than 0.2. Hence an Fe--Si alloy system does not have all characteristics required for a desirable material which can be used as a thermo-electric conversion element.
With an Si--Ge alloy system, the Ge content generally ranges from 20 to 30 atomic %. The material cost of the Ge element is high and the Ge element is prone to segregate, so that it is hard to produce a uniform material. In addition to these problems, there are several drawbacks in characteristics; namely the Si--Ge alloy system exhibits a high value of Seebeck coefficient at high temperature, and its efficiency index, ZT, is about 1.0 at 1,200K since the electric resistance is high although the thermal conductivity of the Si--Ge alloy system is low. As a result, all necessary requirements for a promising thermo-electric conversion element are not met.
The present inventors found that, by adding various alloying elements to Si-based material, the Seebeck coefficient can be equal or higher to that obtained from conventional alloy systems such as an Si--Ge system or an Fe--Si system. More specifically, this novel Si-based alloy system possesses much higher values of the carrier concentration when compared to those found in an Si--Ge alloy system or an Fe--Si alloy system. Based on these fundamental findings, P type semiconductors and N type semiconductors in which various alloying elements are added to Si-based material have been proposed as a promising Si-based alloy system thermo-electric conversion material which exhibits excellent producability, a stable quality, low cost, and high value of efficiency index.
Namely, by adding various amounts of certain types of properly selected alloying elements to the Si-based material in order for the Seebeck coefficient to show the maximum value in a range of the carrier concentration from 10.sup.19 to 10.sup.21 (M/m.sup.3), and by adding elements which are heavier than Ge element to the Si-based material, the present inventors have found that the thermal conductivity can be reduced greatly, resulting in a remarkably improved efficiency index which is much higher than that obtained from an Si--Ge alloy system.
However, there are several additional factors which are important for enhancing the thermo-electric conversion efficiency of the thermo-electric conversion elements in both conventional types and novel Si-based alloy systems. The important factors can include a junction between the metallic electrode components and semiconductors at a PN junction procedure and the condition of joining interface between the semi-conductors and lead wires; in other words, the difference in the Fermi energy (Ef) level between the semi-conductor and metal.
According to the currently employed procedures, the junction between bulk materials is made through the silver solder or transition metallic elements. For manufacturing the junction through the powder metallurgical technique, powders of the P type semiconductor and N type of semiconductor are directly subjected to a press-forming method and joined together. By either way, the thermo-electromotive force is largely affected by the joining conditions.
Since the thermo-electric conversion element is usually exposed to extreme variations in temperature, the joint portion may be cracked or fractured due to the thus-generated thermal stress. Hence, the overall properties of the thermo-electric conversion element is largely influenced by the joining technology. It may be necessary to develop and design a suitable type of joining components corresponding to material types of semiconductors.